1. Field of the Invention
The present invention relates to an image forming apparatus and more particularly to an image forming apparatus of the type forming toner images with dark toner and light toner of substantially the same hue and superposing the toner images on a sheet or recording medium to thereby output an image.
2. Description of the Background Art
It is a common practice with an image forming apparatus of the type forming an image by using a single kind of toner for each hue to attach importance to the density of a solid image. To this end, use is made of toner containing an amount of colorant or pigment great enough to provide a solid image with sufficiently high density. On the other hand, an area tonality scheme has customarily been applied to an electrophotographic apparatus or similar hard copy apparatus, which is configured to form an image on a paper sheet or recording medium, for reproducing a medium-density image or a highlight image.
The area tonality scheme, opposite to a density tonality scheme that varies the density of pixels forming an image, reproduces a medium-density image or a highlight image by reducing the ratio of an area over which toner, or ink in the case of printing, deposits. More specifically, the area tonality scheme is capable of reproducing images lying in the range of from a highlight image to a medium-density image or even a high-density image by reducing the size of a toner deposition area, i.e., the size of dots to such a degree that dots cannot be recognized by eye, so that a person, watching an image, is not aware of the size of the toner deposition area.
Assume that images lying in a range of from a highlight image to a medium-density image are reproduced by the area tonality scheme with toner containing an amount of colorant great enough to implement the desired density of a solid image. This kind of toner will be referred to as dark toner hereinafter. Such a method using dark toner brings about the following problems.
In the case of a highlight image, the dark toner must be evenly deposited on dots in an extremely small amount. However, when it comes to an electrophotographic image forming system that forms a latent image on a photoconductive drum with a laser beam and deposits toner on the latent image for thereby developing it, it is difficult to evenly deposit a small amount of toner on the latent image by accurate control during development. This makes the amounts of toner deposited on dots and therefore image density irregular and therefore, when an image is output by the electrophotographic system using the dark toner, aggravates granularity which is one of important factors determining the quality of the range of from a highlight image to a medium-density image. Granularity of an image itself is well known in the art and will not be described specifically in order to avoid redundancy.
As for the electrophotographic system, granularity cannot be improved in the case of a natural image or similar image whose granularity is critical for the reasons stated above. Consequently, image quality attainable with the electrophotographic system is lower than one attainable with a printing system, an ink jet printing system or similar hard copy system.
Japanese patent laid-open publication No. 2002-91165, for example, discloses an image forming apparatus of the type forming a toner image by repeating development with dark toner and light toner and teaches that a difference in mean amount of charge between the dark toner and the light toner is not greater than 20% or that a difference in weight-mean particle size between the dark toner and the light toner is not greater than 25%. Such a configuration, according to the above document, implements a monochromatic image forming apparatus with a particular relation in characteristic between the dark toner and the light toner that insures a stable monochromatic image with desirable tonality. Also, the above document describes that in the case of a color image forming apparatus using dark toner and light toner for each of colors Y (yellow) C (cyan), M (magenta) and K (black), a particular relation in characteristic between the dark toner and light toner is established that insures color reproducibility and tonality for thereby realizing a stable color image.
Japanese patent laid-open publication No. 11-149207 proposes a system in which the sum of a preselected period of time necessary for the replenishment of a developer and a preselected period of time necessary for suspension is selected to be shorter than a period of time necessary for the operation mode of the shortest operation, and the preselected period of time necessary for the replenishment and the period of time necessary for suspension both end during a single operation mode. The problem with conventional methods in general is that the start-and stop of a drive system are apt to occur during the replenishment of toner. At this instant, the amount of toner replenishment becomes unstable due to the influence of the start-up time of a hopper and the collapse of a heap of toner occurring in a developer conveying section, rendering image density irregular and bringing about fog. The above document describes that the method disclosed therein allows a stable image free from irregular image density and fog to be stably produced at all times.
Japanese patent laid-open publication No. 8-305099 teaches a method that does not replenish, even when a P sensor senses the toner-end condition of a developing unit, toner until image formation ends and then replenishes toner after image formation. It has been customary with conventional methods to drive a toner replenish roller and a sponge roller as soon as the toner end condition of a developing unit is sensed, thereby replenishing toner. This, however, causes image formation and toner replenishment to occur at the same time, so that vibration occurs during the formation of a latent image and disturbs the latent image.
Japanese patent laid-open publication No. 8-227213 discloses an image forming method using toner replenishing means for controlling toner replenishment in accordance with the toner content of a developer sensed and means for sensing the area ratio of a latent image. The image forming method taught in the above document sets the amount of developer replenishment, duration of replenishment and replenishing timing. With this method, according to the above document, it is possible to replenish toner in matching relation to the amount of toner consumed in accordance with image density, thereby insuring stable image density at all times. By contrast, conventional methods have a problem that when images with a great image area ratio and images with a small image area ratio are developed alternately with each other, replenishment cannot catch up with consumption because a certain period of time is necessary for toner replenished to reach a developing sleeve, resulting in an irregular image density distribution.
Japanese patent laid-open publication No. 2002-49191 proposes an image forming apparatus characterized by using dark toner and light toner of substantially the same hue and mainly using the light toner for a range of from a highlight image to a medium image in order to improve granularity. Why such an image forming apparatus improves granularity is presumably that more toner deposits when the light toner is used to form the mesh portion of a highlight image than when the dark toner is used to reproduce a highlight image to a medium-density image. More specifically, the amount of toner for implementing given image density is greater when the light toner is used than when the dark toner is used. Consequently, even when the amount of toner to deposit on the dots of a mesh image increases and is slightly scattered, image density does not noticeably vary. The resulting image is eventually desirable in the aspect of granularity.
However, the image forming apparatus of the type using dark toner and light toner of substantially the same hue as described above has some problems left unsolved, as will be described hereinafter.
Generally, in an image forming apparatus, developing units corresponding in number to the kinds of toners to use are arranged, and each is configured to retain toner and deposits it on a photoconductive drum in accordance with a latent image. Because toner in each developing unit is consumed every time the toner is deposited on a latent image formed on the drum, fresh toner is replenished from a toner replenishing device to the developing unit in such a manner as to maintain the toner content of a developer in the developing unit substantially constant, thereby maintaining image density constant. This can be done by sensing the amount of toner used in the developing unit or directly sensing the toner content of the developer in the developing unit.
When fresh toner is replenished to the developing unit, as stated above, the amount of toner in the developing unit, of course, becomes greater than before the replenishment with the result that the amount of toner to deposit on a given latent image increases. Stated another way, the density of an image to be output varies in accordance with the timing of toner replenishment. Although even a conventional image forming apparatus, in a strict sense, fails to fully prevent image density from varying as a result of toner replenishment, it is used with the variation being confined in a practically acceptable level.
In the image forming apparatus of the type using dark toner and light toner of substantially the same hue to which the present invention pertains, it is necessary to replenish each toner such that the amount of toner in the respective developing unit remains substantially constant. At this instant, the amount of consumption of dark toner and that of light toner is usually not related to each other, so that the time when the dark toner must be replenished and the time when the light toner must be replenished are not related to each other. Consequently, it is likely that the dark toner and light toner are replenished to the respective developing units at the same timing. This brings about the following problem, as determined by experiments.
When the dark toner and light toner are accidentally replenished at the same timing, image density varies between an image output before replenishment and an image output after replenishment more than in a conventional image forming apparatus of the type using a single toner for each hue. Such a difference in the density of an output image is unallowable because it causes a person to feel uncomfortable.
The above problem also arises when dark toner and light toner are used in combination in a color image forming apparatus, e.g., an apparatus loaded with dark magenta toner and light magenta toner, dark cyan toner and light cyan toner or dark yellow toner and light yellow toner. That is, when the dark toner and light toner are accidentally replenished at the same timing, image density or hue varies between an image output before replenishment and an image output after replenishment more than in a conventional image forming apparatus of the type using a single toner for each hue. The colors of the resulting output image critically differ from desired colors.